The present invention relates generally to equipment and operations utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a milling guide having orientation and depth determination capabilities.
It is well known in the art to position a whipstock or milling guide within a tubular structure within a subterranean well, so that a cutting tool, such as a drill or mill, may be guided to cut into the tubular structure. Where this operation is performed from a land rig, or from a rig otherwise secured to the earth's surface, positioning of the milling guide may be accurately accomplished using conventional equipment and techniques well known to those of ordinary skill in the art. However, where the operation is performed from a floating rig, or other rig not rigidly secured to the earth's surface, accurate positioning of the milling guide presents several problems. This is particularly so where an opening must be formed through the tubular structure and aligned with another structure disposed externally to the tubular structure.
For example, it is known to mill an opening through a portion of a liner overlying a whipstock positioned in a lower parent wellbore, in order to provide access and fluid communication between the lower parent wellbore, an upper portion of the parent wellbore, and a lateral wellbore intersecting the parent wellbore. A milling guide may be lowered into the liner from the earth's surface and positioned relative to the whipstock, using the depth from the rig floor or sea floor to correlate the milling guide's position relative to the whipstock's position. Unfortunately, when the rig floor displaces with respect to the earth's surface, such as on a floating rig, it is difficult to correlate the milling guide's position relative to the whipstock's position. This is due to the fact that the milling guide is suspended from the rig, while the whipstock is anchored to the earth.
Past attempts to remedy this problem have been only marginally successful. In one technique, the milling guide is conveyed into the liner below the whipstock suspended from a drill string, and a logging tool is separately conveyed through the drill string and used to determine the depth and orientation of the milling guide relative to the whipstock. The milling guide is rotated as needed to orient it toward the whipstock. The logging tool is then withdrawn from the drill string, and the milling guide is raised as needed to position the milling guide opposite the whipstock.
Of course, the rig may move between the time the desired depth of the milling guide is determined, and the time at which the milling guide is raised relative to the whipstock. Note that a significant delay may occur between these times, since the wireline and logging tool are removed from the well before the milling guide is repositioned. Additionally, the orientation of the milling guide may change due to the drill string being displaced within the well.
From the foregoing, it can be seen that it would be quite desirable to provide a milling guide which has orientation and depth determination capabilities for use on floating rigs, so that the milling guide may be directly positioned relative to a structure disposed within a well. Additionally, such a milling guide, or other similarly improved item of equipment, may be suspended from other types of rigs and used in other operations in which it is desired to position the milling guide or other item of equipment in a well. It is accordingly an object of the present invention to provide such a milling guide and associated methods of positioning the milling guide within a well.